What I thnk about this subject. I'm not convinced that a boxer from the past that might have gone 45 rounds has an advantage over a more modern boxer who only goes 15 or 12 rounds. Diffrent distances require diffrent paces. In track there is a reason the best 100 meeter sprinter is not the best 1500/mile runner who is not the best marathon runner each distance requires a diffrent pace. I also think when comparing boxers from two diffrent ears with diffrent rules you have to asume that the boxer who is placed in to the sittuition would adjust. From crusier weight on down there is no such thing as modern training advantages 160 lbs is 160 lbs. At heavyweight a skilled bigger man is going to have an advantage over a smaller skilled man. This dose not mean he would win but he dose hold an advantage.
ripcity is on the money. Let me add another factor in favor of the modern HW, the evolution of the jab which was pretty nonexistent back in Sullivan's time. Whenever I look at old fight films, I'm amazed at how many old timers didn't jab. The jab is the most important punch in boxing. Louis, Tunney, Ali, Holmes and Holyfield employed an effective jab as the cornerstone of thier offense. So did smaller men such as Robinson, Leonard, Pep and Hearns. An old timer would have a hard time dealing with such a potent weapon. Can you envision any fighter today who isn't taught how to throw a good jab? See what I mean?